[Logan]

I'd use a fertilizer for roses, not for rhododendrons, don't know why roses need less potassium.

 

[Logan]

I buy a food that's made for roses, not anything else, also i put a thick layer of mulch after I've fed them in the spring.

 

[Logan]

Make sure that the cow manure is well rotted but it could have weed seeds in it.
I always use a rose fertilizer and compost.